Many subambient fractional distillations employ a cascade configuration, in which a higher pressure rectification section reboils (exchanges latent heat with) a lower pressure distillation column, and also supplies liquid overhead reflux to the lower pressure column. Thus the driving force for the distillation is provided by pressurizing at least part of the feed to the higher rectification pressure. This provides energy efficiency advantages over the alternative approach--use of an external heat pump to drive a column. The advantages are owing to avoidance of the pressure loss and heat leakage associated with compressing the heat pump vapor, removing the compression heat via ambient cooling, and recycling the compressed vapor back to the cold distillation column.
Although it has long been known that intermediate refluxing of a rectifying section can potentially make the rectification more efficient, and also that increases in distillation efficiency are more important in subambient distillations which are driven by mechanical energy, as distinct from above-ambient distillations which are driven by lower value thermal energy, nevertheless there has been only minimal prior use of intermediate refluxing in subambient distillations. It can be hypothesized that two factors contribute to this situation: first, that in order to realize the full benefit of intermediate refluxing, or indeed any benefit at all, it is necessary provide a fairly precise flowrate of intermediate reflux, relative to the flowrates of the column feed and the overhead reflux. Specifically, the intermediate reflux flowrate should be adjusted so as to obtain a "pinch" (a near-approach between operating line and equilibrium line on the McCabe-Thiele diagram) at the intermediate reflux height at the same time that pinches are also achieved at the feed height and the overhead reflux height. Secondly, there has historically been a difficulty in cascade distillations (also referred to as "dual pressure" or "doubler distillations") in obtaining any amount of intermediate reflux at all, let alone the very narrowly defined optimal quantity.
In the field of cryogenic air distillation, three means have previously been disclosed for obtaining intermediate reflux liquid (liquid air) in a cascade arrangement, with the liquid air subsequently being divided into separate intermediate reflux streams for both the high pressure (HP) rectifier and the low pressure (LP) distillation column.
In one method, disclosed in U.S. Pat. NO. 4670031, the liquid oxygen bottom product from the low pressure column is evaporated at above LP column pressure by exchanging latent heat with about 28% of the supply air, while essentially totally condensing the air.
In a second method a minor fraction of the supply air is totally condensed so as to reboil the LP column bottom, as disclosed in co-pending application 010332 filed Feb. 3, 1987 by Donald C. Erickson now U.S. Pat. No. 4,769,055.
In a third method a minor fraction of the supply air is cooled and then work-expanded to a pressure intermediate to the HP rectifier pressure and the LP column pressure, so as to produce refrigeration, and is then totally condensed against evaporating kettle liquid which is depressurized to the approximate LP column pressure. This method is disclosed in co-pending application 946484 filed Dec. 24, 1986 by Donald C. Erickson now U.S. Pat. No. 4,777,803.
In all three of the above applications it is further possible to additionally compress the minor air fraction en route to total condensation to above the supply pressure, and also to use the expansion work to provide at least part of the additional compression.
The problems with the above three disclosed means of providing intermediate reflux liquid are that the amount of liquid air produced in each instance is dictated by some objective other than obtaning the optimal quantity of intermediate reflux liquid. In the first method, typically some 28% of the supply air must be totally condensed to evaporate about 20.5% of the air as O.sub.2 product. In the second method, some 20 to 24% of the supply air is typically condensed to provide the appropriate quantity of LP column bottom reboil. In the third method, only about 8 to 12% of the air need be expanded and totally condensed to provide the desired refrigeration. In contrast, the optimum distillation efficiency of both the HP rectifier and the LP column rectifying section is achieved when between about 8 and 20% of the supply air is totally condensed and split between the columns; and most optimally (depending upon process variables) about 14%.
The third technique ("AIRPER") overlaps into the optimal range but has the disadvantage of also requiring a liquid air pump.
What is needed, and one objective of this invention, is a means of providing intermediate liquid reflux (totally condensed feed vapor) in optimal amounts (so as to cause the triple pinch condition desired) to both the HP-rectifying section and LP column of a cascaded subambient distillation, while at the same time deriving maximum benefit from the total condensation step.
In many subambient distillations, and increasingly in air distillation, substatial quantities of more than one product are desired. For example, in many oxygen-production processes a substantial quantity of pressurized nitrogen is also desired. One direct benefit of providing optimal intermediate reflux liquid, and another objective of this invention, is the coproduction of the maximum possible amount of pressurized co-product for a given input of compression energy. In one embodiment, another objective of the disclosed invention is to produce at least part of the co-product at a pressure which is actually higher than the supply pressure.